Magnetic circular dichroism (MCD) of cyclic Pi-electron systems will be used to study protonation sites and tautomerism in heterocycles and to characterize substituent effects. Theory suggests that the substituent effects on benzene MCD will be sensitive to solvation and ion-pairing; the possible use of MCD on suitable benzene derivatives as environmental probes will be explored. MCD on matrix-isolated solutes will provide information on vibronic coupling in aromatics such as naphthalene. The successful recent model for the prediction and interpretation of MCD spectra of aromatics in terms of molecular structure will be extended to systems derived from a 4N-electron perimeter, such as quinones, and from (4N+1) and (4N+3)-electron perimeters, such as radical ions. MCD on partially oriented solutes contained in aligned liquid crystals or produced by photoselection in matrices will be measured and analyzed and is expected to yield detailed information on the origin of the MCD effect in Pi-chromophores, in particular, the effects of states other than Pi Pi*.